don't uppercase error messages

a more general version of https://github.com/rust-lang/rust/pull/146080.

after a bit of hacking in [`fluent.rs`](https://github.com/rust-lang/rust/blob/master/compiler/rustc_fluent_macro/src/fluent.rs), i discovered that i'm not the only one that is bad at following guidelines 😅. this pr lowercases the first letter of all the error messages in the codebase.

(i did not change things that are traditionally uppercased such as _MIR_, _ABI_ or _C_)

i think it's reasonable to run a `@bors try` so all the test suite is checked, as i cannot run some of the tests on my machine. i double checked (and replaced manually) all the old error messages, but better be safe than sorry.

in the future i will try to add a check in `x test tidy` that errors if an error message starts with an uppercase letter.

compiler/rustc_driver_impl/messages.ftl

@@ -14,7 +14,7 @@ driver_impl_ice_version = rustc {$version} running on {$triple}
 
 driver_impl_rlink_corrupt_file = corrupt metadata encountered in `{$file}`
 
-driver_impl_rlink_empty_version_number = The input does not contain version number
+driver_impl_rlink_empty_version_number = the input does not contain version number
 
 driver_impl_rlink_encoding_version_mismatch = .rlink file was produced with encoding version `{$version_array}`, but the current version is `{$rlink_version}`
 
@@ -24,6 +24,6 @@ driver_impl_rlink_rustc_version_mismatch = .rlink file was produced by rustc ver
 
 driver_impl_rlink_unable_to_read = failed to read rlink file: `{$err}`
 
-driver_impl_rlink_wrong_file_type = The input does not look like a .rlink file
+driver_impl_rlink_wrong_file_type = the input does not look like a .rlink file
 
 driver_impl_unstable_feature_usage = cannot dump feature usage metrics: {$error}